Composite material leaf springs are known, which springs typically comprise a reinforcing material, for example glass roving or other filamentary solids, in an organic solid such as thermoplastic or thermosetting plastic. Such springs are shown, for example, in U.S. Pat. No. 2,600,843; U.S. Pat. No. 2,829,881; and U.S. Pat. No. 3,142,598. In the past, composite material leaf springs have been used in suspension systems such as, for example, automotive vehicle suspension systems with associated mounting hardware.
Composite material leaf springs are known to provide significant weight savings over comparable metal leaf springs adapted for the same application. The lower weight of the composite material leaf spring is an important advantage in certain application, such as in automotive vehicle suspension systems, where lower vehicle weight translates directly into increased fuel economy. Composite material leaf springs can be manufactured by any of several known methods including filament winding, compression molding, pultrusion, hand lay-up methods or the like or by a hybrid method incorporating some combination of these methods. Filament winding techniques are known to provide several advantages over alternative methods of manufacturing composite material leaf springs. One significant advantage is the lower cost and relative simplicity of filament winding manufacturing techniques. The necessary equipment is well known generally and includes a winding mandrel, a chamber for a resin bath and simple fixtures and devices to guide the filaments through the resin bath, to regulate the amount of resin carried by the filament, to maintain proper tension on the filaments and to orient and guide the filaments onto the winding mandral. All such production equipment is relatively inexpensive in comparison, for example, to production equipment employed in pultrusion (especially for curved leaf springs). In addition, the filament winding process itself can be less complex than other methods such as pultrusion. Certainly, filament winding provides far greater productivity than hand lay-up techniques.
Filament winding typically involves coating or impregnating glass roving, yarn or the like with a resin composition comprising liquid resin and a suitable curing agent for the resin. The roving can be dipped into the liquid mixture or otherwise brought into contact with it. One or more layers of the impregnated roving is then wound onto a mandrel having a suitable configuration to provide a preform of the desired product. In the case of filament winding a leaf spring, the resin-impregnated filaments can be wound into an annular trough in the radial edge of a wheel-shaped mandrel being rotated on an axle. The preform can then be cured, optionally with application of heat, pressure or both, over a period of time to yield the desired product. Numerous classes of resins suitable for use in filament winding techniques are known to the skilled of the art. These include, for example, epoxy resins, silicone resins, polysulfides, polyurethanes, vinyl ester resins and polyester resins and the like.
Composite material leaf springs have been suggested in a wide variety of shapes and dimensions. Production of leaf springs by a filament winding method presents certain design constraints, most significant of which is that the leaf spring will necessarily have a substantially constant total cross-sectional area. To obtain such constant cross-sectional area leaf springs with suitably low spring rates and better spring efficiency, it is necessary to flare the ends of the leaf spring, that is, to make the ends of the spring wider (width being the dimension of the upper and lower surface of the leaf spring perpendicular to the longitudinal axis of the leaf spring) and less thick (thickness being the dimension of the leaf spring between the upper and lower surfaces, perpendicular to the longitudinal axis of the leaf spring). Typically, the spring is given a narrow, thick cross-section in the central area of the spring, that is, in the axle attachment area of the spring. This is generally necessary in view of the high bending moments experienced by that portion of the spring. To provide a lower spring rate, the cross-sectional shape of the leaf spring is generally made to gradually become wider and less thick toward the end portions of the leaf spring. Less thickness is sufficient in the end portions since the leaf spring experiences smaller bending moments there. The lower spring rate advantageously provides increased flexibility and durability.
Further, while it is known that a certain degree of flare at the end portions of the spring can be provided during the filament winding process, a critical limitation in this regard is the need to maintain tension on the resin-impregnated filaments as they are wound onto the winding mandrel. Filament tautness during the winding process ("tautness" meaning, at least, absence of slack; typically, for example, the filaments as a whole are wound under about 20 lbs. tension) is needed to properly align the filaments to produce springs of adequate strength, durability, etc. However, since the filaments are under such tension and since the filaments after exiting the resin bath are guided to the winding mandrel from a stationery or substantially stationery guide fixture, the filaments cannot be made to follow a severely flared contour in the winding mandrel. It will be apparent that filaments wound into an annular trough having a severely flared contour, that is into a portion of a trough which rapidly widens, would be pulled toward the axial center line of the winding path as the winding mandrel continued to rotate and, hence, would not follow the flared contour. The filaments would become concentrated toward the center of the leaf spring as winding proceeded. Even if the resin flowed sufficiently to fill in the flared contour without filaments, such leaf spring would have significantly reduced strength and durability. For this reason, filament wound leaf springs in the past, unless subjected to secondary shaping steps, have provided little or no flare and, if flared, they have generally provided only substantially straight line flare from the axle attachment portion in the center of the spring to the spring attachment eyes. Of course, even where the leaf spring does have such straight line flare, the maximum width of the flare would necessarily be limited to the maximum permissible width of the eye of the spring. The eye at each end of a filament wound leaf spring generally is relatively narrow (as noted above regarding the central portion of the spring, the axle attachment portion) to provide sufficient thickness and strength, since this portion is used for mounting the leaf spring to the vehicle chassis. Due to such constraints, filament wound leaf springs in the past have provided less than optimal spring efficiency, less than optimal energy storage capability per unit mass of composite material of the leaf spring and, thus, have been more expensive and have weighted more than desirable for many applications.
A composite material leaf spring can be given a flared configuration by compression molding or like step following the filament winding process. Such added process steps, however, are disadvantageous since they involve added expense, time and complexity. In addition, the deformation or dis-alignment of the filaments of the leaf spring by such secondary shaping steps can significantly reduce the strength and durability of the finished spring. Accordingly, there is a recognized need for a filament wound leaf spring which can be fashioned either with or, preferably, without secondary shaping methods. More specifically, there is a need for a filament wound composite material leaf spring which, given any particular maximum length and maximum width design constraints, provides optimal spring efficiency to minimize the weight, size and cost of the spring, that is, a leaf spring having such configuration as to provide optimal spring flexibility consistent with the strength of the composite material and optimal energy storage capability per unit mass of composite material of the spring. In U.S. Pat. No. 3,900,357 a composite material spring is suggested. The composite material spring of this reference, however, is not a filament wound leaf spring. It would not lend itself to be made by a filament winding technique since it is not of constant cross-sectional area. In any event, it is not flared at the end portions and, hence, would not provide a spring or optimal spring efficiency, i.e., it would not provide a spring of the greatest possible percentage of the theoretical energy storage per unit mass of composite material.
Various metal leaf springs for suspension systems, such as automotive vehicular suspension systems, are widely known and used, for example, that of U.S. Pat. No. 3,490,758. A filament wound composite material leaf spring following the configuration of such known metal leaf springs, however, would not provide optimal energy storage capability per unit mass of composite material of the leaf spring. Other known metal leaf springs such as, for example, that shown in U.S. Pat. No. 3,439,400, are not of constant cross-section and, thus, could not be produced using filament winding techniques.